In optical disks such as DVDs and CDs, analog signals are treated. Thus, requirements for light source noise are extremely severe.
On the other hand, as noise generated by a semiconductor laser that is used to record/reproduce data on/from an optical disk, there are return light noise and mode hopping noise. Both of them give serious interference on a reproducing signal.
The return light noise is caused by that because a part of a beam reflected on the optical disk returns to the inside of the semiconductor laser, the optical disk operates as an external reflective mirror and mode competition occurs between the disk and an original internal resonator and oscillation occurs. The mode hopping noise is noise that is generated when an axial mode changed by that the length of the resonator of the semiconductor laser itself varied by a change in temperature.
In an optical disk recording/reproducing apparatus, there is a problem that laser noise increases by a return light from an optical disk and a change in temperature of a semiconductor laser and it has a bad influence on a reproducing signal. As a technique to reduce the laser noise, there is a laser noise cancel (LNC) system in that noise components in a laser beam directly monitored are cancelled out from an optical signal that was subjected to modulation by an optical disk (for example, Japanese Patent Laid-Open number 2002-352459).
Here, a conventional optical disk apparatus in the LNC system will be described by FIG. 4.
Referring to FIG. 4, an optical head section 40 is composed of a laser diode (LD) 41, a collimator lens (CL) 42, a polarization beam splitter (BS) 43, a ¼ wavelength plate (QWP) 44, an objective lens (OL) 45, a collective lens 46, a light receiver for reproducing signal (RFPD) 47 and a light receiver for monitoring light source (FPD) 48.
In such optical disk apparatus, a laser beam emitted from the laser diode 41 passes through the collimator lens 42, and enters into the polarization beam splitter 43. A part of the laser beam which passed through the polarization beam splitter 43 passes through the ¼ wavelength plate 44 and is converted into a circularly polarized light, and then it is collected on an optical disk 50 by the objective lens 45. This laser beam is modulated by recorded information in the optical disk 50, and then it passes through the objective lens 45 and the ¼ wavelength plate 44 again. The laser beam is returned to a linearly polarized light by the ¼ wavelength plate 44 and enters into the polarization beam splitter 43. After the laser beam is reflected on its plane of polarization separation, the reflective light enters into the light receiver for reproducing signal 47 through the collective lens 46. On the other hand, a part of the laser beam emitted from the laser diode 41 is reflected on the plane of polarization separation of the polarization beam splitter 43, and the reflective light enters into the light receiver for monitoring light source 48.
Further, the optical signals detected by the light receiver for reproducing signal 47 and the light receiver for monitoring light source 48 are converted into electric signals. They are amplified by amplifiers 51 and 52 respectively so as to be equal to each other in noise level, and then an LNC signal by that only laser noise components were canceled out from an RF signal in an arithmetic circuit 53 formed by a differential amplifier circuit or the like is outputted.
Here, the polarization beam splitter 43 has polarization properties, and the ¼ wavelength plate 44 that restrains the effect of a return light is inserted in an optical path on which the RF signal is extracted. Therefore, the ratios of the TE wave to the TM wave of the laser beams received by the light receiver for reproducing signal 47 and the light receiver for monitoring light source 48 are sometimes different. In this case, relativity of noise between the TE wave and the TM wave of the laser beam emitted from the laser diode 41 is low. Therefore, even if all of the noise can be canceled out in the arithmetic circuit 53 by equalizing the level of one of noise components (for example, a TE component) in the RF signal received by the light receiver for reproducing signal 47 to the level of a noise component in an FPD signal (the same TE component) received by the light receiver for monitoring light source 48 by the amplifiers 51 and 52, the noise of another noise component (a TM component) cannot be completely canceled out because their levels are different.
Specifically, in the laser diode 41, as modes showing a polarization direction of a laser beam emitted from the laser diode 41, as shown in FIG. 5, there are two modes of a TE mode in which the electric field is polarized to the direction parallel to an active region layer (the direction vertical to the thickness direction of the active region layer), and a TM mode in which the electric field is polarized to the direction vertical to the active region (the direction parallel to the thickness direction of the active region layer). In their modes, noise is generated mutually not correlatively. Thus, there is a problem that a canceled amount lowers in optical systems having polarization dependency.